Conventionally, in order to improve the speed change efficiency of a derailleur, a first spring is interposed between the chain guide and the movable member and a second spring is interposed between the base member and the fixing member, so that the first and second springs provide tension to the chain and are balanced with each other to set the chain guide in position with respect to each sprocket.
The second spring biases the pantograph mechanism clockwise with respect to the fixing member, in other words, biases the guide pulley radially outwardly from the multistage sprocket assembly, and the chain guide, even when shifting the chain, has a tendency to move under a spring force of the second spring so that an interval between the guide pulley and the smaller diameter sprocket becomes larger than that between the guide pulley and the larger diameter sprocket. As a result, the proper interval, even when set with respect to the larger diameter sprocket, is too large as the interval for the smaller diameter one. Especially, when a difference in diameters of both sprockets is larger, the interval between the guide pulley and the smaller diameter sprocket is too much larger, thereby creating a problem in that the speed change efficiency deteriorates.
In order to solve the above problem, it is proposed that the second spring is reduced in its spring force so as to set the chain guide to be reduced in the interval between the guide pulley and the smaller diameter sprocket. The reduced interval, however, in some cases may cause an interference of the guide pulley with the larger diameter sprocket when the chain is shifted thereto.
In the construction where the guide pulley is controlled in position by use of a balance between both the springs, the control capacity is smaller and the spring force of each spring should correspond to a difference in tooth number between the larger and smaller diameter sprockets. Also, the first spring increases in its resilience as the difference in tooth number increases, whereby the second spring also should increase its spring force for preventing the guide pulley from contacting with the larger diameter sprocket when the chain is shifted thereto. As a result, when the first and second springs are set to obtain the interval of the predetermined length between the guide pulley and the larger diameter sprocket, and when the chain is shifted to the smaller diameter sprocket, the second spring of increased spring force makes the interval between the guide pulley and the smaller diameter sprocket too large, which is an unavoidable problem.
On the other hand, the Japanese Patent Publication No. Sho 42-23,485 discloses a derailleur in which connecting pins for connecting the linkage members and base member and the movable member and linkage member, are slanted with respect to a plane perpendicular to the axis of the multistage sprocket assembly, the chain guide being forcibly moved radially of the sprocket assembly in the plane parallel to the axis of the same.
This construction, however, requires the chain guide to move in a specific direction so that, when the sprocket assembly is changed, the not-uniform intervals between each sprocket may deteriorate the speed change efficiency.